Evaluating the relationship between arterial blood pressure changes and indices of pulse oximetric plethysmography

The finger plethysmographic waveform of pulse oximeters is a qualitative indicator of fingertip perfusion. This waveform has been used to assess the depth of anesthesia. Its cyclical changes associated with mechanical ventilation have also been used to detect changes in blood volume under normotensive conditions and has revealed that minimal normotensive hypovolemia can cause a significant increase in the delta-down component of this waveform. Hypovolemia may be associated with hypotension; the latter may be due to causes other than hypovolemia. Because the effects of the hypotension on plethysmographic waveform have not been evaluated, it may be difficult to detect hypovolemia in these conditions by inspecting a plethysmogram. Therefore, we performed this study to evaluate the effect of normovolemic hypotension on characteristics of plethysmographic waveform in 33 adult patients undergoing general anesthesia with controlled hypotension. The delta-down and ventilatory systolic variation components were increased significantly with decreases in systolic blood pressure. The result of this study shows that the effect of pharmacologic hypotension on the plethysmographic waveform of pulse oximeter is similar to that of minimal hypovolemia. Therefore, blood volume may be inaccurately assessed by the inspection of ventilatory-induced cyclical changes of pulse oximetric waveform in the presence of hypotension.

The cyclical respiratory-induced changes in the amplitude of the pulse oximeter waveform can be used to detect normotensive hypovolemia. This study shows that hypotension produces the same effect. Therefore, in hypotensive conditions, we cannot determine the presence of hypovolemia.